Agglomerating partially dehydrated gel-derived pseudoboehmitic alumina to form strong porous spheres

ABSTRACT

Gel-derived crystalline alumina containing at least 40 percent pseudoboehmite and having low cationic and anionic impurity levels is partially dehyrated to an LOI of 22-34 percent, ground to at least 85 percent minus 325 mesh, shaped into spheres by conventional agglomerating devices while adding sufficient water to rise the total water content to 52-65 percent. These shaped spheres, without any ageing treatment, are then heated to a temperature in the range of 350*-650*C for a time necessary to produce the desired strength. The resultant spheres are strong and have a high total porosity, at least 20 percent of which consists of pores in the 120-800 A size range. They are useful as desiccants, active alumina and catalyst supports.

United States Patent [1 1 Beiding et al.

[ 51 Jan. 30, 1973 [75] Inventors: William A. Beiding; Robert B.Emerson; Raymond L. Williams, all of Baton Rouge, La.

{73] Assignee: Kaiser Aluminum & Chemical Corporation, Oakland, Calif.

22 Filed: Jan.27, 1971 21 App1.N0.:ll0,3l8

[52] U.S.Cl. ..264/1l7,23/313,252/448,

252/463, 423/122, 423/131 [51] lnt.C l. ..B0lj ll/44,C01f7/02 [58] Fieldof Search .....264/63, 117; 106/65; 23/141, 23/143, 313; 252/448, 463;423/122, 131

3,009,885 1l/l961 Bertolacini ..23/l43 2,881,051 4/1959 Pingard ..23/3133,222,129 12/1965 Osment et al. ..23/141 3,480,389 11/1969 Graulier..23/143 3,520,654 7/1970 Carr et al. ..23/143 Primary Examiner-Donald.1. Arnold Assistant Examiner-John H. Miller Atl0rney-Paul E. Calrow,Harold L. Jenkins and Andrew E. Barlay [57] ABSTRACT Gel-derivedcrystalline alumina containing at least 40 percent pseudoboehmite andhaving low cationic and anionic impurity levels is partially dehyratedto an L01 of 22-34 percent, ground to at least 85 percent minus 325mesh, shaped into spheres by conventional agglomerating devices whileadding sufficient water to rise the total water content to 52-65percent. These shaped spheres, without any ageing treatment, are thenheated to a temperature in the range of 3 50-650C for a time necessaryto produce the desired strength. The resultant spheres are strong andhave a high total porosity, at least 20 percent of which consists ofpores in the 120-800 A size range. They are useful as desiccants, activealumina and catalyst supports.

5 Claims, No Drawings AGGLOMERATING PARTIALLY'DEHYDRATED GEL-DERIVEDPSEUDOBOEHMITIC ALUMINA TO FORM STRONG POROUS SPHERES BACKGROUND OF THEINVENTION This invention relates to gel-derived alumina shapes ofimproved physical and mechanical properties and to a method of makingthese shapes. More particularly, the present invention relates tothermally stable gelderived alumina shapes characterized by high surfacearea and porosity, low attrition loss and high crushing strength and toa method of making these shapes.

Gel-derived aluminas are widely utilized, for example, as catalysts,catalyst supports and also as desiccants. These aluminas generallyexhibit relatively high surface area and porosity and are thermallystable at the high temperatures commonly utilized in catalytic reactionsand in drying. For many applications, it is important to employ shapedproducts, as these products exhibit higher strength properties, reducedresistance to flow of gaseous compounds through a reactor filled withthe catalyst, and are easier to load and remove from reactors thanpowdery or granular materials. The shaped products in general have manydesirable properties characteristic of granular products; for example,spheres, nodules and hollow cylinders of the prior art have high surfacearea and porosity and also thermal stability. Nevertheless, due to themicrocrystalline and poorly organized crystal structure, the shapesheretofore formed from gel-derived aluminas lack the combination of suchproperties with the strengths desired and requiredin most fields ofapplications.

Recognition of the desirability of using spherical products and the lackof physical strength of the existing products led to earlier suggestionsall directed to the improvement of strength'properties. Thus, it hasbeen suggested in U.S. Pat. No. 3,380,933 that gelderived alumina shapesof increased strength be produced by dehydration of hydrous alumina,followed by shaping and sintering at temperatures between 1500C andl800C. The shaped and sintered product is shown to exhibit strengthimparted by the sintering step; however, due to the use of hightemperatures within the ranges indicated, the active surface associatedwith the pores having pore diameters in the 120-800 A range issignificantly reduced, thus making the product less desirable for use asa catalyst base.

Another process for shaping gel-derived alumina particles is describedin U.S. Pat. No. 3,264,069, wherein it is suggested to dehydrategel-derived alumina to a loss on ignition of about percent by weight,followed by grinding, shaping, aging and calcination to a low loss onignition. While the resultant product as described possesses improvedhardness and abrasion resistance, the dehydration treatments and agingutilized materially affect other properties of the balls, for example,phase stability. Even more significantly, porosity ofthe material isreduced,.thus resulting in a dense product which has less porosity inthe important pore size range desired in the catalyst industry. All theprior art efforts indicated the need for a-gelderived alumina shapewhich not only exhibits high crushing strength, low abrasion lossgenerally less than about 1 percent, high attrition resistance,generally less than about 70 percent, but also high porosity and highsubstantially active surface coupled with thermal stability. It has nowbeen surprisingly discovered that such a product can be made fromgel-derived aluminas without sacrificing any of the properties requiredby the catalyst industry.

BRIEF SUMMARY OF THE INVENTION Gel-derived alumina shapes of highstrength and high porosity are made from pseudoboehmitic aluminacontaining at least about 40 percent by weight of pseudoboehmite. Thepseudoboehmitic alumina is dried after precipitation to a water contentbetween about 22-34 percent by weight, followed by grinding to aparticle size range, wherein at least 85 percent of the ground particlespasses through a screen having openings of 0.044 mm. The groundparticles are then shaped in the presence of water added in an amountdetermined by the formula of W W,W,, wherein W is the water added to theshaping, W, is the total water content of the alumina during shaping andW, is the water content of the ground alumina prior to shaping, andwherein W, is maintained between about 52-65 percent by weight of thealumina. The shaped particles can then be thermally treated to obtaincrushing strengths in excess of about 7 kg, total porosities in excessof about 0.6 cc/g with surface areas in excess of about 220 m /g.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides agel-based alumina shape, possessing thermal stability, high surface areaand high porosity, further characterized by high attrition resistanceand high crushing strength.

In accordance with the process of the present invention, gel-derivedaluminas are partially dewatered, followed by grinding to apredetermined particle size. The ground particles are then shaped withwater while controlling the water content of the alumina during shapingwithin closely controlled limits. The preferred gelderived aluminasforuse herein are pseudoboehmitic aluminas characterized bya'pseudoboehmite content of at least about 40 percent by weight asdetermined by X-ray diffraction analysis in comparison to an essentiallypure standard pseudoboehmite exhibiting an X- ray diffraction peak inthe 6.5-6.8 A range as measured by copper Ka radiation at 14.5, 2Bangle.

The gel-derived alumina utilized in the present invention can beadvantageously prepared by the neutralization of an alkali aluminatesolution with a strong acid or an acidic aluminum salt, such as AI(NO;,)AI (SO,) or AlCl The preferred alkali aluminate solution is a sodiumaluminate solution, while the strongacid can be selected from the groupof acids including I-ICI, HNO and H An advantageous process for thepreparation of pseudoboehmitic alumina containing a high percentage ofpseudoboehmite is described in U.S. Pat. No. 3,630,670. In accordancewith the process of the above-referenced application to N. Bell et al, asodium aluminate solution having an alumina (AI O to caustic (expressedas Na CO weight ratio from about 0.7 to about 0.9 is neutralized with anaqueous nitric acid solution (50-200 grams/liter HNO under carefullycontrolled temperature and pH conditions. The resultant neutralizationproduct is filtered and washed and the hydrous alumina product ofsubstantially pseusolutions and acids or acidic aluminum salts alsoresulting in a gel-derived alumina containing at least 40 percent byweight of pseudoboehmite alumina can be utilized for the presentprocess.

ln general, the freshly precipitated hydrous alumina is contaminatedwith both anionicand cationic impurities. These impurities should beremoved prior to shaping, as they may affect the performance of thealumina when used as a catalyst support. impurities most commonlyencountered include Na,0 or 14,0 and anions such as Cl, S0, or NO,,depending on the acid or acidic aluminum salt employed to neutralize thealkali aluminate solution.

Suitable purification processes include washing of the precipitatedhydrous alumina after filtration with deionized water and/or repulpingof the filter cake in deionized water. After these purificationtreatments, the impurity level of the pseudoboehmitic alumina issuitably low; it generally exhibits a cationic impurity level of lessthan about 0.1 percent and an anionic impurity level of less than about0.2 percent.

The wet purified alumina is then prepared for shaping. Due to the highfree and bound water content of the hydrous alumina, successful shapingcannot be accomplished without reduction in this water content. Mostsuitably, a controlleddrying treatment is employed which will reduce thewater content of the hydrous alumina to such an extent as to allowpreparation of shapes. Such drying can be conveniently accomplished indrying devices known in the art, such as spray-driers. It has now beensurprisingly discovered that in order to attain the heretofore describedcombination'of optimum properties, namely, high crushing strength, highporosity and high surface area, certain critical controls must bemaintained. Not only is it essential to individually control watercontent during dehydration and during shaping within critical limits,but it is also necessary to maintain distinct relationships betweenscreen analysis of the ground pseudoboehmitic alumina, water content ofthe partially dehydrated alumina andwater content during shaping. Inother words, the novel combination of the final properties of the shapedmaterial is a function of all three above defined variables. I

Thus, under the term control, for the purposes of this invention thefollowing operating steps are understood:

a. water content control of the effluent alumina from the dryer;

b. particle size control after partial dehydration and prior to shaping;

c. water content control during shaping.

a. Water content control during drying Regardless of the type of dryingequipment utilized, the water content of the dried gel-derived aluminashould be controlled to be. between about 22 and about 34 percentbyweight, preferably between about 24-32 percent by weight. The hydrousalumina, which has been filtered and washed, usually contains about 80percent by weight water. Thus, the drying step employed removes asubstantial portion of this water content. The rate of water removal isnot considered to be critical; nevertheless, it should be set in amanner so as to accomplish the desired goal, i.e., reduction of thewater content to about 32 percent, or below, but not below about 22percent by weight without, however, prolonged residence time in thedrying equipment.

b. Particle size control The dried pseudoboehmitic alumina is usuallyrecovered from the drier in a finely divided state. Nevertheless, inorder to assure uniformity in the forming operation, it is recommendedthat the dried alumina particles be ground to a particle size wherein atleast 85 percent, preferably 90 percent, of the alumina particles passthrough a screen having openings of 0.044 millimeters (equivalent to a325 mesh U.S. Standard Sieve). It has been observed that grinding of theparticles to the aforementioned particle size improves the attritionresistance, crushing strength and, in general, the physical propertiesof the final shaped product. It is believed that grinding of theparticles will eliminate voids and consequently allows better compactionof the shapes without, however, a reduction in actual porosity orsurface area. Regardless of the explanation provided above,the describedcontrol of the grinding operation is essential in combination with theother two controls to achieve the unique product.

c. Shaping operation The dried and ground alumina particles are thensubjected to a shaping operation. The shapes produced may be of anydesired configuration; for example, spheres, spheroids or nodules.Regardless of the configuration of the shapes to be produced inaccordance with the present invention, the water content,i.e., thequantity of water introduced to effect shaping, should be carefullycontrolled. It was found that in order to obtain shapes of .excellentphysical properties, the water added to the shaping operation should notexceed an amount defined by the following formula:

W,= W, W, wherein W, is water added to shaping, W, is total watercontent of the alumina during shaping and W, is water content of theground gel-derived alumina, all expressed in weight percent and based onthe Al,0 content of thepseudoboehmitic alumina. It was found thatexcellent results can be achieved when W is between about 52-65 percentby weight.- Thus, for example, when the water content of the groundalumina is about 28 percent by weight prior to shaping (W,) the waterquantity to be added to the shaping (W,) varies between about 24-37percent by weight. .Water contents are determined by calcining the driedand ground pseudoboehmitic alumina at l000C for lhour .to obtain itstotal free and bound water content, which [is designated and usedhereinafter as loss on ignition (L01). Within the limits given, i.e.,total water contents of 52-65 percent by weight, it has been found thatthere is a correlation between the water content employed in the shapingstep of the invention and the bulk density of the gel-derived aluminaparticles. It was found that if the bulk density of the aluminaparticles is in the range of about 25-35 lbs./ft. (400-560 kg/m), totalwater contents in the range of about 52-58 percent produceoptimumresults, while for bulk densities below about 25 lbs/ft (400 kglmtotal water contents of 56-65 percent provide the desired end results.

To obtain best results, i.e., shapes exhibiting optimum physicalproperties, it is recommended that the water content of the dried andground alumina be ascertained prior to shaping. It is also possible toadjust the drying equipment in such a manner as to consistently producethe pseudoboehmitic alumina of predetermined water content, whichgenerally does not change during the grinding step. Small variations inthe water content of the dried alumina, i.e., 1-3 percent by weight,will only slightly alter the required water addition in the shapingstep; thus, once the drying is conducted in a consistent manner, onlyperiodic moisture checks are required.

The initiation of shaping can be accomplished by any of the well knowntechniques in devices, such as rotating pans, cylinders and drums, butunder steady state operating of the shaping device it is important tocontrol the ratio of water feed to alumina feed to achieve uniformproduction of alumina of the described critical water content. Othermethods of shaping can also be utilized provided the water to aluminaweight percentages described above are maintained.

After the shapes have been formed, they are removed from the formingapparatus. Generally, the formed shapes are subjected to a thermaltreatment or activation treatment to fully develop the strength andsurface properties of the shaped, gel-derived alumina. Thermaltreatments in the range of 350650C are usually employed for theactivation.

The thermally treated shapes can be directly employed as desiccant oractive alumina, or, if desired, impregnated with solutions of metallicsalts to form catalyst compounds. In many instances, admixture with themetallic salts can be accomplished prior to the grinding step; grindingwill cause uniform distribution of the catalytic agents throughout thealumina. The admixture can then be subjected to the aforesaid shapingoperation, provided the limits in water content are strictly adhered to.Another mode of impregnation or admixture with the catalytic agents canbe accomplished directly during shaping. In this instance, the metallicsalts can be either added in an aqueous solution as the nodulizingliquid or in dry form directly to the forming apparatus. In each mode ofadmixture, water content control must be exercised in accordance withthe principles hereinbefore described.

It was found that gel-derived alumina shapes, when processed inaccordance with the present invention, will exhibit high crushingstrength, generally in excess of about 7 kg (l5 lbs.), high totalporosity, usually in excess of about 0.6 em /g, wherein at least percentof the total porosity is provided by pores in the 120-800 A pore sizerange, an attrition loss below about 70 percent, and an abrasion loss ofless than about 1 percent, a combination of properties hithertounobtainable by conventional methods.

The following Tables and Examples will further illustrate the novelprinciples of the present invention:

Example I Pseudoboehmitic alumina containing about 90 percent by weightpseudoboehmite and containing about 82 percent by weight water (free andbound) as determined by heating to l000C for 1 hour was dried in a spraydrier to a water content of about 23 percent by weight. The driedproduct was then ground to a particle size wherein about 90 percent byweight of the particles passed through a screen having screen openingsof 0.044 mm (US. Standard screen 325 mesh). The ground particles werethen added in increments to a cylindrical rotating pan having an insidediameter of about 76 cm and a depth of about 23 cm. The tilt of the panwas kept constant during shaping at about 28 from the vertical while thepan was rotated at approximately 40 rpm. Water was introduced inincrements into the pan through nozzles directed against the wall of thepan in a manner as to obtain a total water content in the alumina ofabout 56 percent. The shapes which had a spherical appearance andretained their pseudoboehmite content were then removed and, withoutaging, thermally treated at about 400C for about 1 hour. The spheroidswere then analyzed, with the results shown in Table 1.

Example ll Pseudoboehmitic gel-derived alumina ground to a particle sizewherein 90 percent passed through a screen having openings of 0.044 mm,containing in excess of about 80 percent by weight pseudoboehmite andhaving a combined free and bound water content of about 30 percent byweight, was introduced at a uniform rate into the forming devicedescribed in Example l. Water was also introduced into the formingdevice so as to maintain a total water to alumina weight percent ofabout 57.2 percent. The shaping was accomplished at the same rpm andtilt described in Example I. The formed shapes were then removed fromthe device, activated without aging at about 400C for 1 hour. Theactivated shapes were then analyzed and the results are shown in Tablell.

Experiments were conducted byvarying one of the variables describedabove, while maintaining the other variables between the critical limitsshown. Thus, a shaped product was prepared by drying a pseudoboehmiticalumina containing in excess of 80 percent by weight of pseudoboehmiteto a water content of about 19 percent by weight. The dried alumina wasthen ground to a particle size range wherein about percent by weight ofthe ground particles passed a screen having openings of 0.044 mm. Theground alumina was then shaped as described in Example I by addingsufficient water to obtain a water content of about 56 percent of theweight of the alumina content of the shapes. The shapes were thenthermally treated at about 400C for about 1 hour and analyzed. Theshapes thus produced, while retaining high porosity and high surfacearea, exhibited an average strength below about 4.5 kg.

In another experiment the particle size range was changed, while thewater content of the partially dehydrated pseudoboehmitic alumina andthe water content of the shapes were kept within the critical limitsdisclosed. The pseudoboehmitic alumina was ground to a particle sizerange wherein about 60 percent by weight passed a screen having openingsof 0.044 mm. The thermally treated shapes exhibited high surface areaand a crushing strength near the minimum achievable by the presentinvention; however, the attrition loss of the shapes was totallyunacceptable, being in excess of 90 percent.

Table 1 Physical Characteristics of Gel-derived Alumina Shapes Made fromPseudoboehmitic Alumina Surface area m'lg 339 Total porosity cc/g 0.82Porosity in 120-800 A range cc/g 0.17 Bulk density kg/m 467 Loss onignition k 6.2 Crushing strenigth kg 8.8 Attrition loss 3 8.3 Abrasionloss I; 3 0.4 Appearance and size Spheroids of 3.2 mm av. diam. Thepseudoboehmitic alumina starting material for the shapes contained about30.8 percent water after spray-drying as determined by loss on ignitionand 22.8 percent water as determined by drying at 104C for about 1 hour.(1) Crushing Strength A sample of spheroids having an average diameterof 3.2 mm was placed on a screen having openings of 3.36 mm and 25spheroids retained in the openings of the screen were removed fortesting while both the undersized and oversized spheroids werediscarded. Each of the 25 spheroids was placed in a "Chatillon ModelHTCM Crushing Strength Tester operated at a speed of 3. The crushingstrength was determined by readin the pressure in kg required to crushthe spheroid. The results for the spheroids were averaged and reportedas crushing strength in kg.

(2) Attrition Loss ('11) A sample of 30 grams of spheroids of 3.2 mmaverage diameter was placed into a 1000 ml Erlenmeyer having a hole of25.4 mm in the bottom. This hole was covered with a screen havingopenings of 1.41 mm. A rubber stopper was inserted at the top of theflask and in the rubber stopper a metal inlet nozzle was inserted havingan internal diameter of 0.5 cm. The flask was then inverted, fixed inthis position and connected to an air pressure regulating system. Airwasadmitted for a period of minutes at a rate of 10.4 mlhour andsubsequently the material was screened on a screen having openings of1.41 mm. The amount remaining on the screen was then weighed. Theattrition loss was calculated as follows: Attrition loss (30.0 Final wt.ofsample)/( 30) X 100 (3) Abrasion Loss (5) A wei hed amount ofspheroids (av. diameter 3.2 mm) were tapped on a O-TAP machine for 30minutes. The material was then screened and the material passing throughthe screen (0.595 mm. openings) was weighed and recorded as abrasionloss.

3.2 mm av. diam.

It can be observed from the data shown above that shapes of highporosity, strength and surface area can be made without the necessity ofactivating the pseuoverall su erior ro erties.

It is to e und rs 00d that the process of the present invention iscapable of producing shapes of varying appearance and size withoutchanges in the critical parameters described in detail above.

What is claimed is:

l. A process for making shapes of high strength, controlled bulk densityand high porosity from gel-derived alumina which comprises:

a. partially dehydrating a gel-derived, crystalline alumina containingat least about 40 percent by weight of pseudoboehmite and having ananionic impurity content less than about 0.2 percent and a cationicimpurity content less than about 0.1 percent to a loss on ignition ofabout 22-34 percent by weight;

. grinding the partially dehydrated alumina to a particle size rangewhereinat least percent by weight of the ground particles pass, througha screen having openings of 0.044 mm;

. shaping the ground alumina in the presence of water added in an amountdetermined by the formula of W, W, W,, wherein W, is the water added tothe shaping, W, is the total water content of the alumina during shapingand W, is the water content of the ground alumina prior to shaping, andwherein W, is maintained between about 52-65 percent by weight of thealumina;

. recovering the shaped alumina; and

.- thermally treating without ageing the shaped alumina at temperaturesin the range between about 350650C for a time sufficient to develop acrushing strength of at least about 7 kg, an attrition loss below about70 percent and an abrasion loss of less than about 1 percent.

2. Process according to claim 1, wherein the gelderived alumina isdehydrated to a total water content of about 24-32 percent by weight.

3. Process according to claim 1, wherein the total water content (W,) ofthe shaped alumina is about 52-58 percent of the weight of the aluminacontent of the shape, and the bulk density of the ground, gelderivedalumina is between about 400-560 kg/m.

4. Process according to claim 1, wherein the total water content (W,) ofthe shaped alumina is about- 56-65 percent of the weight of the aluminacontent of the shape, and the bulk density of the ground, gelderivedalumina is less than about 400 kg/m.

5. Process according to claim 1, wherein the gelderived alumina isground to a particle size range wherein at least about percent by weightpasses through a screen having openings of0.044 mm.

i i t l

1. A process for making shapes of high strength, controlled bulk densityand high porosity from gel-derived alumina which comprises: a. partiallydehydrating a gel-derived, crystalline alumina containing at least about40 percent by weight of pseudoboehmite and having an anionic impuritycontent less than about 0.2 percent and a cationic impurity content lessthan about 0.1 percent to a loss on ignition of about 22-34 percent byweight; b. grinding the partially dehydrated alumina to a particle sizerange wherein at least 85 percent by weight of the ground particles passthrough a screen having openings of 0.044 mm; c. shaping the groundalumina in the presence of water added in an amount determined by theformula of Ws Wt - Wg, wherein Ws is the water added to the shaping, Wtis the total water content of the alumina during shaping and Wg is thewater content of the ground alumina prior to shaping, and wherein Wt ismaintained between about 52-65 percent by weight of the aluminA; d.recovering the shaped alumina; and e. thermally treating without ageingthe shaped alumina at temperatures in the range between about 350*-650*Cfor a time sufficient to develop a crushing strength of at least about 7kg, an attrition loss below about 70 percent and an abrasion loss ofless than about 1 percent.
 2. Process according to claim 1, wherein thegel-derived alumina is dehydrated to a total water content of about24-32 percent by weight.
 3. Process according to claim 1, wherein thetotal water content (Wt) of the shaped alumina is about 52-58 percent ofthe weight of the alumina content of the shape, and the bulk density ofthe ground, gel-derived alumina is between about 400-560 kg/m3. 4.Process according to claim 1, wherein the total water content (Wt) ofthe shaped alumina is about 56-65 percent of the weight of the aluminacontent of the shape, and the bulk density of the ground, gel-derivedalumina is less than about 400 kg/m3.